[PATCH v6 net-next 10/14] tcp: accecn: retransmit SYN/ACK without AccECN option or non-AccECN SYN/ACK

chia-yu.chang
2025-11-14T07:14:17Z

For Accurate ECN, the first SYN/ACK sent by the TCP server shall set the
ACE flag (see Table 1 of RFC9768) and the AccECN option to complete the
capability negotiation. However, if the TCP server needs to retransmit such
a SYN/ACK (for example, because it did not receive an ACK acknowledging its
SYN/ACK, or received a second SYN requesting AccECN support), the TCP server
retransmits the SYN/ACK without the AccECN option. This is because the
SYN/ACK may be lost due to congestion, or a middlebox may block the AccECN
option. Furthermore, if this retransmission also times out, to expedite
connection establishment, the TCP server should retransmit the SYN/ACK with
(AE,CWR,ECE) = (0,0,0) and without the AccECN option, while maintaining
AccECN feedback mode.

This complies with Section 3.2.3.2.2 of the AccECN specification (RFC9768).

Signed-off-by: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>

---
v6:
- Use new synack_type TCP_SYNACK_RETRANS and num_retrans.
---
include/net/tcp_ecn.h | 20 ++++++++++++++------
net/ipv4/tcp_output.c | 4 ++--
2 files changed, 16 insertions(+), 8 deletions(-)

diff --git a/include/net/tcp_ecn.h b/include/net/tcp_ecn.h
index a709fb1756eb..57841dfa6705 100644
--- a/include/net/tcp_ecn.h
+++ b/include/net/tcp_ecn.h
@@ -649,12 +649,20 @@ static inline void tcp_ecn_clear_syn(struct sock *sk, struct sk_buff *skb)
}

static inline void
-tcp_ecn_make_synack(const struct request_sock *req, struct tcphdr *th)
-{
- if (tcp_rsk(req)->accecn_ok)
- tcp_accecn_echo_syn_ect(th, tcp_rsk(req)->syn_ect_rcv);
- else if (inet_rsk(req)->ecn_ok)
- th->ece = 1;
+tcp_ecn_make_synack(const struct request_sock *req, struct tcphdr *th,
+ enum tcp_synack_type synack_type)
+{
+ // num_retrans will be incresaed after tcp_ecn_make_synack()
+ if (!req->num_retrans) {
+ if (tcp_rsk(req)->accecn_ok)
+ tcp_accecn_echo_syn_ect(th, tcp_rsk(req)->syn_ect_rcv);
+ else if (inet_rsk(req)->ecn_ok)
+ th->ece = 1;
+ } else if (tcp_rsk(req)->accecn_ok) {
+ th->ae = 0;
+ th->cwr = 0;
+ th->ece = 0;
+ }
}

static inline bool tcp_accecn_option_beacon_check(const struct sock *sk)
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 5fa14a73d03f..c6754854ad09 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1106,7 +1106,7 @@ static unsigned int tcp_synack_options(const struct sock *sk,

if (treq->accecn_ok &&
READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_ecn_option) &&
- req->num_timeout < 1 && remaining >= TCPOLEN_ACCECN_BASE) {
+ synack_type != TCP_SYNACK_RETRANS && remaining >= TCPOLEN_ACCECN_BASE) {
opts->use_synack_ecn_bytes = 1;
remaining -= tcp_options_fit_accecn(opts, 0, remaining);
}
@@ -4004,7 +4004,7 @@ struct sk_buff *tcp_make_synack(const struct sock *sk, struct dst_entry *dst,
memset(th, 0, sizeof(struct tcphdr));
th->syn = 1;
th->ack = 1;
- tcp_ecn_make_synack(req, th);
+ tcp_ecn_make_synack(req, th, synack_type);
th->source = htons(ireq->ir_num);
th->dest = ireq->ir_rmt_port;
skb->mark = ireq->ir_mark;
--
2.34.1